Aquarium filtration is a bit of a mystery to most people. There is a common misconception that the filter should take care of overfeeding and keep the water perfectly suitable for fish. Manufacturers make a big deal out of it. Most of the new filters coming to market are large, complex and expensive. The companies making them, lead you to believe that if you are having problems with your fish, then it's probably due to the lack of filtration. You may be surprised to learn that the amount of filtration is the least likely cause of most problems. In this article, I hope to clear up the mystery and make this an easy concept to understand. Keeping your aquariums clean and suitable for fish is quite easy as you'll see.
Let's get down to basics. You can filter water with three basic methods. There is chemical, mechanical and biological filtration. Most filter systems involve a combination of at least two of these and some use all three. We are often led to believe that all are necessary, yet in my opinion only one is really important, and effective in most filters. Now let's take a more detailed look at each and how much sense it makes to incorporate them into your filtration methods.
With chemical filtration You use an item like carbon or zeolite to remove an impurity. The chemical reaction that takes place is usually very short lived and its effectiveness lessens rapidly from the very beginning of its use. In my opinion, unless you want to do an extraordinary amount of maintenance on a continual basis, this type of filtration is suitable only as a temporary measure. It's great for emergencies, removing medications from the water or trying to reduce sudden spikes of toxins. It's good to have some of these items on hand, but don't bother to incorporate them into your daily filtration system. In general, doing so would be a waste of time and money.
Mechanical filtration: This involves the trapping and removal of waste particles. In concept, this is a great idea. In reality, most filters cannot do this in a manner that is effective or convenient for the aquarist. Most mechanical filters do a great job of trapping some particulate matter, but unfortunately they don't get it all. They have a tendency to move the water too fast, thus breaking the particulate matter into smaller pieces. The very small pieces tend to become suspended. These suspended micro particles contain the dangerous heterotrophic bacteria that can potentially cause great harm to our fish. The bacteria should be kept away from our fish, but these suspended particles do the opposite. They are in the water column and can be very harmful.
Filters that move water through the aquarium at higher speeds, cause this problem to become worse. Small waste particles are the enemy. Filters that move water too quickly and those that create a large amount of small bubbles, break these particles into even smaller pieces and will actually cause this bacteria to become an even greater problem. To encourage small waste particles to settle in the filter chamber, water movement must be the slow enough to cause the particles to settle. This is very difficult to achieve with most power filters and canister filters. Very large aquariums or aquaculture systems will generally have large filter systems that contain proportionately large settling chambers, where these fine particles can be eliminated from the water column.
In addition to removing these particles from the water column, they need to be removed from the bottom of the tank. The fins of fish often touch the bottom, and the waste particles that settle here can cause problems when the fish rubs against them. Therefore, a filter needs to draw from the tank bottom and anything it doesn't get, must be removed through siphoning. After waste has been on the tank bottom for more than a day, it has been largely broken down by the heterotrophic bacteria and turned into an unsightly, but relatively harmless mulm. At this point, it's only danger is that it will slowly increase the dissolved solids in the water, and will contribute to higher nitrates. However, it will do this whether it's on the tank bottom or sitting in a filter. Unless you clean your filter a couple time/day, it won't make much difference.
Biological filtration: This is the process by which nitrifying bacteria break down ammonia and nitrites. I will not cover the basics of biological filtration. That is detailed in many other sources. Just realize that it is easy to have adequate nitrifying bacteria in aquariums containing ornamental fish. In intensive aquaculture, it is common to raise as much as 1 lb of fish per gallon of water, with relatively small biological filters. That would be equivalent to raising somewhere around 100-150 adult angelfish in a 20 gallon tank. In such a system containing angelfish or other ornamentals, problems from dissolved organics and heterotrophic bacteria would destroy the fins or kill the fish long before ammonia or nitrites became a problem. A surprisingly small biological filter can handle the ammonia produced in the average aquarium containing ornamental species. So, although biological filtration is very important, it's also very easy to provide with a small inexpensive filter. The only requirement is that the filter does not clog so the nitrifying bacteria has constant exposure to oxygenated water, and that the filter does not move the water too fast, producing the dangerous micro-particles of waste that are so harmful.
Tying it all together: As you have probably surmised by this point, chemical filtration is not practical or effective for most aquarists. In addition, mechanical filtration is normally performed in a manner that can actually be detrimental. Unfortunately, most aquarists rely heavily on these and are not aware of the best way to utilize them. In fact, some of the most expensive filters can also be some of the least effective.
One of the oldest filters is an undergravel filter. Water flows through the gravel, and in the process will create a very effective biological filter. If it is set up to have the flow go down through the gravel and then up a lift tube, the gravel will collect waste, making it an effective mechanical filter. It often gets an undeserved bad rap. When maintained properly, they are very effective. Add in the fact that they are very inexpensive to purchase and operate, and you have one of the better choices for an aquarium that contains a substrate.
When setting one up, place the filter plates on the bare tank bottom. Then cover them with a layer of polyester batting or even better - an inch or two layer of reticulated foam. The foam will prevent the substrate from falling into the filter plates and it will also provide greater surface area for nitrifying bacteria. Cover with a substrate that is fine enough that no food particles can fall beneath the surface of the substrate. This will allow the easy siphoning of uneaten food that cannot be trapped. When doing a water change, use a gravel cleaner to remove particulate matter. Do this to no more than one half of the substrate during any one water change. Vary the location of the substrate cleaning with each water change. Using this technique, I've maintained beautiful, healthy aquariums for more than 20 years without ever having to add any additional filters or perform any other maintenance.
For breeding operations or the raising of fry, bare bottom tanks should be used. In these situations, nothing beats a simple sponge filter or whole-tank foam filters for effectiveness and ease of maintenance. However, not all foam filters are equally good. You must choose one with a pore size appropriate for the fish size being kept. The object is to keep the pores from getting clogged with food or fish feces. It should provide adequate surface area for nitrifying bacteria. The sponge type should allow easy rinsing of the filter. Yet, it must also allow space for the settling of organic debris.
If one sponge filter isn't enough, use more or switch to whole-tank foam filters. Slow to moderate flow rates are essential. The smaller the filter, the slower the flow rate must be. The inside of the sponge becomes the settling chamber. Too much flow, and the settling chamber will not work. It is important that the sponge filter lifts water from the bottom of the tank. It not only makes it easier to get particles off the bottom and into the filter, but it turns the water over in the tank more efficiently for greater gas exchange. Therefore, foam filters should sit flat on the bottom. Those that are on a pedestal, may create dead spots in the aquarium, and are the worst at trapping particles that make it to the tank bottom.
A note about filter size: Filters are not sized for a particular number of gallons of water. They work by consuming ammonia and nitrites produced by a particular bio-load. The bio load consists of the total mass of fish and heterotrophic bacteria in the tank. It matters not if the tank is large or small, filters have to be sized accordingly to the number and size of fish in relation to age, water temperature, pH and a few other factors. It is something you can only figure out for a given situation through experience. As long as the water isn't moving too fast in the tank, it doesn't hurt anything to over-filter, which is why we are so partial to whole-tank foam filters.
So far, providing the needed filtration sounds fairly simple, but don't get too excited. One of the more important aspects of filtration can't be performed perfectly by any filter and is usually done manually. That is, the aquarist must periodically remove fish waste and detritus with water changes, and they must also occasionally rinse the filters to keep them from clogging. Water changes are what is used to remove harmful dissolved organics and nitrates. Most aquarists worry about ammonia and nitrites. However, they are easily controlled and seldom a problem for anyone other than a beginner with poor husbandry practices. Dissolved organics and heterotrophic bacteria are the real concerns, yet they are almost impossible for an aquarist to detect. It is critically important to keep them at low levels. Water changes are the most effective way to do this.
How to rinse sponge filters: Gently squeeze the sponge into fish-safe water (we use water taken out of the aquarium from a water change). Do not rinse it too thoroughly. You don't want to wash all the nitrifying bacteria out of it. Never clean them in a washing machine or dishwasher. This will essentially kill all the good nitrifying bacteria and render your filter useless.
Water changes can even be used to remove uneaten food, but hopefully your fish husbandry is good enough that uneaten food doesn't exist. The frequency needed for water changes will vary greatly with fish density, temperature, amount of food being put into the tank, pH and a few other factors. It's better to err on the side of more water changes. You can perform too few, but never too many.
It should be a relief to know that through the combination of properly designed foam filters, correct feeding, and adequate water changes, you can filter an aquarium better and at lower cost than any other practical method. Don't fall prey to the hype surrounding expensive aquarium filters. There exists some very effective, sophisticated and expensive central filtration systems designed for hatcheries, however for practical filtration on individual aquariums, nothing works better than the simple filters recommended here.